The present invention relates to nuclear fuel assemblies, and more particularly, to grid strips which form an egg crate type fuel rod support grid in a pressurized water nuclear fuel assembly.
Conventionally, pressurized water nuclear reactors have a core of nuclear fuel assemblies, in which the fuel rods are supported and spaced relative to each other, by spacer grids. Each cell of the grid utilizes a system of fuel rod support features including low stiffness springs and opposing high stiffness arches to contact, locate, and stabilize the fuel rod. At the time of fabrication of the fuel assembly, the geometry of each grid and associated cell is intended to provide lateral forces between the fuel rod and the support features. This type of fuel rod support is represented by U.S. Pat. Nos. 4,803,043, 4,879,090 and 5,188,797. Typically, the separation distance in the as built grid, between each spring and its opposing arches, is less than the outside diameter of a new fuel rod. Thus, when a rod is inserted into a grid cell, the spring is compressed and the rod is held between the support features with a preload force. Since these support or contact features project into the flow stream that passes through the cell during operation in the reactor, the rod support structure should present a low cross section to minimize pressure drop losses.
The effect of the reactor environment on the initial grid/rod interface is to cause the preload force to diminish through a combination of short-term and long-term mechanisms, followed by gap formation in cases where the force drops to zero. Short-term mechanisms include the effects of initial heatup of the fuel (partial relaxation of forming stresses in the grid strip and possible yielding of the grid strip due to reduced yield strength at higher temperatures) and initial pressurization of the reactor (spring compression is reduced because the fuel rod diameter decreases due to differential pressure). Long-term mechanisms include spring stress relaxation due to temperature and neutron flux, fuel rod cladding diametral creepdown, and, in the case of spacer grids made of Zircaloy, radiation growth of the grid cell.
Full scale flow tests have shown that inadequate rod support by the grid results in excessive rod vibration, leading to wear and possibly rod failure. These tests have shown that the key to limiting rod wear is the prevention of significant gaps between the grid contact features and the rod. Based on the mechanisms described above there are two general ways to minimize gaps. One can lessen the creepdown of the fuel rod cladding or improve the short-term and long-term responses of the grid features to reactor operating conditions. It is the latter approach which is used in the present invention.